1. Field of Invention
This invention relates generally to probe assemblies for testing integrated circuit (IC) devices whose contact pads are deployed in a predetermined pattern around the IC periphery in a common plane, and more particularly to an assembly of this type which includes a probe card with test terminals connectable to a plurality of probe fingers arranged in a matching pattern in the form of spring contact fingers, whereby when an IC device to be tested is raised to bring its contact pads into engagement with the spring fingers, this action causes the fingers to flex upwardly and inwardly, and in doing so to scrub the contact pads and thereby ensure effective electrical contact therewith, so that the IC may be tested while still on the silicon wafer.
2. Status of Prior Art
Many complex electronic circuits previously regarded as economically unfeasible and impractical are now realizable in integrated circuit (IC) form. The fabrication of a single-crystal monolithic circuit involves the formation of diodes, transistors, resistors and capacitors on a single microelectronic substrate formed on a silicon wafer. The circuits are applied to the wafer by photolithography, each wafer containing an array of identical integrated sections. The wafer is then sliced into "dice," so that each die or chip carries a single integrated circuit. In a typical IC chip, input and output terminals, power supply and other circuit terminals are created by metallized contact pads, usually deployed along the margins of the chip. The geometry of the chip is either square or rectangular, and the marginal locations of the contact pads thereon depend on the circuit configuration and the available marginal space, usually being laid out in a line of uniformly spaced pads along each edge of the chip.
In order to ensure that the various circuits in each IC chip are functioning properly, so that the chip will operate reliably in the electronic apparatus in which it is included, one must test the chip before it is installed. The packaging of an IC chip is relatively costly and time consuming, and because a significant number of IC chips fail the test and have to be discarded, it is desirable that each chip be tested before being placed in its package.
An early test probe for testing semiconductor devices such as beam-leaded transistors or integrated circuit chips is shown in U.S. Pat. No. 3,867,698, in which sets of converging test probe leads are embedded in the lower surface of a square annular frame of thermo plastic material. The inner ends of the leads extend in cantilevered fashion into a central opening, to terminate in flat and smooth tips which were pressed against the leads projecting from the integrated circuit ship. While such a test probe was suitable for beam leaded or packaged IC's, it is not suitable for testing circuits in situ on the silicon wafer before slicing into dice. For this, a needle test probe card evolved.
An unpackaged IC chip having no pins or leads can be electrically connected to IC chip-testing instrumentation by means of a needle-type test probe card, one well known form of which is disclosed in Evans U.S Pat. No. 4,382,228. The Evans card includes an opening providing access to the IC chip being tested, the opening being surrounded by a ring of spaced conductive platforms on which are anchored blade-like needle holders. Extending from each holder is a deflectable needle to engage a respective contact pad on the IC chip being tested.
In a subsequent Evans U.S. Pat. No. 4,719,417, the test probe card is provided with double-bent needles such that when the IC chip to be tested is raised upwardly to cause its contact pads to engage the tips of the needles, further upward movement of the chip, known as "overdrive", gives rise to lateral displacement of the tips along the surfaces of the contact pads. This results in a scrubbing action serving to break through any oxide film formed on the pads that would otherwise interfere with effective electrical contact.
Although needle probe cards have served the IC industry well for many years, they suffer two principle limitations: First, the pitch of the contact pads in present day IC's is very small, typically 0.005 to 0.004 inches, and approaching 0.003 in the foreseeable future. This minuscule spacing causes severe crowding of manually assembled and adjusted needles, and consequent electrical shorting and difficulties in repairing. Secondly, the higher operating frequencies of present day IC's demands a "ground plane" which extends as closely as possible to the point of contact between the needles and the IC pads, a feature absent in needle probe cards. The ground plane provides the probe array with a characteristic impedance which matches the impedance of the integrated circuit test apparatus. Future operating frequencies will be higher still, and thus make needle probe cards increasingly unsatisfactory.
In order to provide a test probe card suitable for IC chips having a high density of contact pads, Gangroth et al, U.S. Pat. No. 4,649,339, makes use of a flexible dielectric film having a sheet of copper laminated thereto that is etched to define conductive leads or traces which terminate in probe contacts. These probe contacts are patterned to engage the contact pads of high density VLSI or other integrated circuits. To bring about engagement between the probe contacts on the flexible film membrane and the contact pads on the IC chip, the film is arranged as a diaphragm covering an air chamber. When pressurized air is supplied to this chamber, the film is then flexed outwardly to conform to the surface of the IC chip. The practical difficulty with this arrangement is that the bulging film fails to scrub. Moreover, users generally object to having any part of the device other than the probe contacts making physical contact with the wafer under test.
An alternate to the needle probe cards with the scrubbing action provided by probe needles, is the use of so-called "bump technology", using contact pressure alone to effect good electrical contact throughout the oxide film. Rath U.S. Pat. No. 4,758,785 discloses a motorized lift system for raising an integrated circuit device upwardly against the probe contacts on a flexible film, above which is a resilient pressure pad of silicone- rubber. The practical difficulty with this arrangement is that the rubber pressure pad does not reliably ensure planarization and/or scrub of the probe contacts, and without such planarization and scrub, proper testing cannot be effected.
It must be borne in mind that when an IC wafer whose contact pads lie in a common plane is raised by a lifting mechanism toward a probe test card in which the probe contacts are formed on a flexible film, the wafer may then be caused to assume a slight angle relative to the plane of the film. As a consequence, contact pressure between the probe contacts and the contact pads will not be uniform, and this may militate against effective testing of the device.
To provide a high-density test probe assembly capable of accurately testing an IC device having a multiplicity of contact pads deployed in a common plane, Evans et al U.S. Pat. No. 4,975,638 discloses an assembly having a membrane formed by a flexible film of dielectric material, the membrane including a square contactor zone from whose comers extend radial slots to define suspension quadrants. Probe contacts are formed in a matching pattern on the face of the contactor zone. The suspension quadrants are supported at their ends on corresponding branches of a mounting frame, while the integral contactor zone sags below the frame which surrounds a central port in a printed circuit board, the port exposing the contactor zone to the IC device to be tested.
In the Evans et al '638 assembly, the probe contacts or bumps formed on the face of the contactor zone are connected to conductive traces running along the suspension wings. Since the assembly is put to repeated use, the probe bumps are eventually worn down. Also, they may lack adequate salience. Because the dielectric film on which the bumps are formed may be imperfect and exhibit minor projections, salience is necessary to raise the bumps above these projections.
We have also found, however, that contact bumps formed on the face of the contactor zone, even if given greater salience, tend to collect debris in the course of testing procedures. Moreover, compliance of the individual bumps is inadequate. Any seemingly minor or ordinary difference in adjacent bump heights require forces that are unacceptably high in order to obtain good mechanical contact on the recessive bumps by reason of insufficient elasticity in the membrane carrying the bumps.
We have also found that scrub is unreliable when limited to one direction, which is the case with devices having contact bumps formed on the face of the contactor zone. Such scrub may be achieved by various means, for example, by a cantilevered mounting arm angled downward that produces a slight lateral unidirectional excursion of the entire bump pattern when overdrive results in vertical excursion of the probe assembly. The net resulting lateral movement depends on the friction between the bumps and the pads on the device under test, and the bearing play found in the chuck of the testing machine. Such bearing play may range from virtually zero to dimensions that match or exceed the desired length of scrub. In the latter case, no scrub is produced.
Efforts to achieve the desirable scrubbing action of prior art test probe cards with flexible needles, as disclosed in the aforementioned earlier Evans U.S. Pat. Nos. 4,382,228 and 4,719,417, while also achieving the advantage of photolithographic techniques to produce the needles, led to development of a probe assembly which included a film membrane in which spring contact fingers were formed as extensions of traces running along the membrane. Cantilevered spring fingers were obtained by processing a multi-ply laminate having a copper ply serving as the ground plane, laminated with an adhesive layer to a dielectric of polyimide in turn laminated to a thin sheet of spring alloy copper. The lattermost ply was etched to define spring fingers and traces integral therewith and the spring fingers subsequently bent to obtain sharp contact fingers supported on the laminated membrane. This probe assembly and technique are disclosed in a co-pending application entitled "Probe Assembly for Testing Integrated Circuit Devices" in the names of Arthur Evans, Joseph R. Baker, and Jack Lander Ser. No. 08/001759 filed Jan. 7, 1993, and assigned to the present assignee.
The aforementioned pending application describes an improved probe assembly adapted to test an integrated circuit (IC) device whose contact pads are deployed in a predetermined pattern in a common plane. The assembly includes a film membrane having a planar dielectric contactor zone and an array of suspension quadrants radiating from this zone in a manner similar to Evans et al U.S. Pat. No. 4,975,638. The wings are clamped at their ends by a mounting ring surrounding a port in a printed circuit board which exposes the contactor zone suspended below the board to the IC device to be tested, the ring being secured to the board. Deployed in a matching pattern on the exposed face of the contactor zone and cantilevered therefrom to assume an angle with respect thereto are spring contact fingers that are extensions of traces running along the wings of the membrane to leads on the printed circuit board. These leads extend to respective test terminals on the board which are connectable to electronic instrumentation for conducting tests on the IC device. When the IC device is raised to bring its contact pads into engagement with the spring fingers, this action causes the fingers to flex inwardly and upwardly, and in doing so to scrub the contact pads and thereby ensure effective electrical contact.
While the aforementioned device achieves good scrub action, the test membrane is expensive to produce, delicate to handle, and experiences difficulty in achieving planarity. Because of the requirement to bend the spring alloy copper fingers after attachment and etching, it is difficult to attain uniform bending so that the finger tips are aligned. It is desirable to have all finger tips planar within 0.001 inches. When co-planarity of the finger tips must be achieved on all four edges of the contact zone of the membrane, uniform contact with the integrated circuit pads is difficult to achieve.
Also since the fingers are formed by bending, fatigue life becomes a factor in the life of the test probe membrane. Fatigue life could be dramatically improved by providing a finger assembly formed by photolithography and etching techniques, which does not require bending the etched trace extensions.
And also, because the ratio of finger thickness, width, and pitch is limited by photolithographic manufacturing processes, and these limits, in turn, dictate a finger length that results in adequate spring force, the compromised balance of resulting spring fatigue life and electrical performance generally does not provide a number of test cycles that is economic.
Although all of the contact pads are co-planar on the integrated circuit, it will be appreciated that the contact pads are usually arranged in straight lines and disposed on the four peripheral edges of the integrated circuit. A simplified test probe assembly with improved means to separately adjust the probe finger tips with respect to each separate line of pads, where they are arranged in linear sets of spaced contact pads on the IC periphery, would be desirable.
Accordingly, one object of the present invention is to provide an improved probe assembly for testing integrated circuits while on the silicon wafer before further processing and separating the dies or chips.
Another object of the invention is to provide an improved test probe assembly for connecting respective test probe fingers to closely spaced, linear sets of co-planar IC contact pads with good scrub action.
Another object of the invention is to provide an improved test probe assembly utilizing probe finger arrays formed by photolithography and etching techniques and having good fatigue life.
Still another object of the invention is to provide an improved adjustment device for achieving adjustment of a line of probe fingers supported on a flexible laminated member.
Another object of the invention is to provide an arrangement for obtaining precise alignment of probe fingers through the use of independent leaf springs each supporting multiple probe fingers.
Still another object of the invention is to provide improved probe arrays having a ground plane providing a characteristic impedance that matches the impedance of the integrated circuit test apparatus.